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Nuda is a class of ctenophores or comb jellies. The class contains a single family, Beroidae, with two genera, Beroe and Neis, and the group is more commonly referred to as the "beroids". They are distinguished from other comb jellies by the complete absence of tentacles, in both juvenile and adult stages. Beroe is found in all the world's oceans and seas, and the monotypic Neis occurs only near Australia; all beroids are free-swimmers in the plankton.
Some members of the diverse genus Beroe may occasionally attain a length of up to 30 centimetres (12 in), though most species and individuals are less than about 10 cm; Neis cordigera is among largest species in the class. The sack-like body of the Beröe species may be cylindrical in cross section, or compressed to varying amounts by species, while Neis is somewhat flattened and characterized by a pair of trailing gelatinous "wings"that extend beyond the aboral pole[vague].
Like other comb jellies, the body wall of nudans consists of an outer epidermis and an inner gastrodermis, separated by a jelly-like mesoglea. The mesoglea has pigments that give many nudan species a slightly pink color; Neis cordigera may be yellowish or a deep orange-red.
Nudans have a very large oral cavity, allowing them to swallow prey whole. While swimming, and particularly while pursuing prey, it can close this mouth like a zipper so that the nuda maintains a streamlined profile. The nuda "zips" its mouth closed from each end, and the "lips" seal shut by forming temporary inter-cellular connections. The whole ctenophore nerve network is within the "lip" of the Nuda of about 40 nerve cell Fort records (Neurit), this bundle of compacted.
The palate is caused by the interplay of three different types of muscle open and closed: In the mesoglea lost both ring around the longitudinal axis of the animal and arranged radial, radially outwards from the inside running muscle fibers; they interact with longitudinal, so längsseitig oriented fibers, which the epidermis.
Nudans have only smooth muscle fibres.
Directly inside the mouth opening can be found in the lining of the gap characteristic finger-like excesses, Macrocilia. In this first 1844 of JGF Will described and 1965 by George Adrian Horridge with her today named structures are conical shaped by a common plasma membrane surrounded bundles of two to three scourges (cilia), which, as in the comb plates of ribs to a functional unit connected. The individual, between 35 and 60 micrometres in diameter long and 5 to 10 micrometres thick cilia together in a hexagonal cross-sectional structure, and are perpendicular to the direction of common connected by crossbars, a separate membrane, which they divide their neighbor scourges would have it they are not. Each cilium shows the typical eukaryote construction of nine external and two internal microtubules.
The scourge cells, in which the cilia from individual basal bodies arise, to the Schlundauskleidung by an average of thirty degrees to mundabgewandten side and, stacked on top like roof tiles. Thus, the macrocilia always point in the direction of the interior of the throat. This is where is the effective strike, which is prey parts in synchronized waves like a conveyor belt transported in the stomach and the throat muscles promoted this process.
Macrocilia run in a three toothed top of this is stiff enough to the soft outer wall to rip even larger prey like other ctenophores; proteolytic enzyme penetrate into the resulting wounds, make the victim then quickly escape.
Macrocilia also function as teeth, and can be used in the same way as the tentacles of Tentaculata.
From the main drag from stomach like all ctenophore channels in all regions of the body. Each rib will comb through its own meridional canal furnished directly below this location. He owns at the Nuda species many fine ramified excesses, some of which deal with the gap unite, but others end up blind. To that end mouth running the meridional canal in a circle around the mouth lying ring channel.
As with all ctenophores, is at the mundabgewandten side Gleichwichtsorgan, statocyst, including the clock for the ribs arranged on the ridge platelets determine. The shock pulse, unlike in the Lobata, is not through the ridge between platelets oblique scourge forwarded tapes, but the fact that the individual comb platelets in turn fall, like dominoes, the subsequent platelet influence.
Immediately below the statocyst is an inlet lined with sensory cells, which contains epithelial tissue has to mouth out in two opposite narrow bands scourge continues, in the throat level of ctenophore run and as a polepieces. You call a strike by its water flow on the statocyst shows and are used by the Nuda-Arten at its border with finger excesses, the papillae occupied whose function is still unclear, but probably also sensory perception.
Unlike other groups of ctenophores put them to continue their movement almost exclusively comb ribs.
Nudans feed on free swimming animals with soft bodies, primarily from other ctenophores, certainly larger than they themselves can be. They actively hunt for prey, which they usually devour whole. Some species use their macrocilia as teeth to take smaller chunks from the prey.
In the late 1980s the ctenophore species Mnemiopsis leidyi was introduced into the Black Sea, probably through ballast water, which led to the collapse of the local anchovy population. In 1997 another ctenophore species arrived--Beroe ovata, a predator of Mnemiopsis leidyi. The Beroe population underwent an initial explosion, until the numbers of both ctenophores stabilized. Nevertheless, both Mnemiopsis leidyi and Beroe ovata remain today in the Black Sea. The same phenomenon is occurring at the beginning of the 21st century in the Caspian Sea.
All species reproduce sexually, and have both female and male gonad. Although no detailed figures are available, it is assumed that self-fertilization is the exception among nudans.
The fertilized eggs hatch into miniature versions of the adult animal, rather than distinct larval forms. They already lack the tentacles and Tentakelscheiden, otherwise they are similar to the Cydippea stadum.
There are no known fossil nudans, so the phylogenetic evolution of the group from the comparison with other modern representatives of Rippenquallen tapped to be. In the traditional system, the Nuda form a class distinct from the Tentaculata, which all have at least rudimentary tentacles. This division, after provisional results of morphological and molecular studies, however, probably does not reflect the actual relationships within the ctenophores.
Although the scheme is still in the river and does not have an independent assessment of the available evidence, seems to be a family from the order Cydippida, Haeckelidae, in the closer relationship of Nuda species to belong neither Cydippida still Tentaculata would therefore cladistic taxa included not all offspring of their joint Vorfahrens and would therefore is not recognised as a valid taxon in modern schemes.
The monophyly of Nuda itself, is however widely accepted, due to the complete lack of tentacles, and the presence of macrocilia as a common secondary feature, or synapomorphy.
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